The invention relates to a bone screw having a threaded section and a head and a receiving part at the head end for receiving a rod to be connected to the bone screw, the receiving part possessing an open first bore and a substantially U-shaped cross-section having two free legs provided with a thread and a second bore at the end opposite to the first bore, whose diameter is greater than that of the threaded section and smaller than that of the head and which forms the seat for the head, and a nut or screw working together with the thread.
Such a bone screw is disclosed, for example, in U.S. Pat. No. 5,672,176. In the known bone screw the head is of spherical segment-shaped construction. The bottom of the first bore adjacent to the second bore is likewise of spherical segment-shaped construction so that the spherical head lies on this spherical section. The plane going through the bounding edge is oriented at right angles to the axis of the first bore and the mid-point of the second bore coincides with the axis of the first bore. By this means it is achieved that the threaded section possessing the head is pivotable in a predetermined angle of generally up to 25xc2x0 about the axis of the first bore so that even after screwing the threaded section into a vertebral segment orientation of the receiving part receiving a rod is possible. At the same time the size of the pivot angle is limited to the extent that the second bore as a function of the diameter of the head must not exceed a certain size so that the head still has an adequate hold in the receiving part.
The use of such bone screws is something of a problem in the region of cervical vertebrae. In this case, due to the small dimensions of the cervical vertebrae, it is necessary that the screws must always be pivoted to one side and upwards, a greater degree of pivoting being necessary than is the case in the larger thoracic vertebrae and lumbar vertebrae.
The aim of the invention is to provide a bone screw which permits a larger pivot angle. This task is solved by a bone screw having a screw member 1 possessing a threaded section 2 and a head 3, a receiving part 5 at the head end for receiving a rod to be connected to the bone screw, wherein the receiving part 5 has an open first bore 6 and a substantially U-shaped cross-section having two free legs provided with a thread, and a second bore 7 at the end opposite to the first bore 6 whose diameter is greater than that of the threaded section 2 and smaller than that of the head 3, and a seat for the head 3 and a nut or screw acting together with the thread, characterised in that viewed relative to the axis of the first bore 6 the edge bounding the free end of the second bore 7 is of asymmetric construction.
Refinements of the invention are characterised in that (i) the bounding edge has a countersink 10 in an angular region, (ii) the normal to the plane going through the bounding edge is inclined to the axis of the first bore 6, and (iii) the mid-point of the plane enclosed by the bounding edge is offset relative to the axis of the first bore 6.
In a preferred embodiment of the invention, a bone screw has a screw member 1 possessing a threaded section 2 and a head 3, a receiving part 5 at the head end for receiving a rod to be connected to the bone screw, wherein the receiving part 5 has an open first bore 6 and a substantially U-shaped cross-section having two free legs provided with a thread, and a second bore 7 at the end opposite to the first bore 6 whose diameter is greater than that of the threaded section 2 and smaller than that of the head 3, and a seat for the head 3 and a nut or screw acting together with the thread, characterised in that on its neck adjoining the spherical segment the screw member 1 has a recess or sink 16.
Further features and practical advantages of the invention emerge from the description of exemplified embodiments with reference to the figures.